The present invention relates to optical media, and in particular, to optical media for data or information storage having a uniformly distributed array of indicia, for example, pits, which are available to provide timing, fine tracking and address information.
Optical media such as optical disks, compact disks and data disks are manufactured to close tolerances. For example, disks which have a center hole placed with high precision reduce radial run out or departure from concentricity. Also disks are produced with little or no substantial radial distortion or elliptical stretching. As the tolerances have been improved the storage capacity of such devices has increased.
Eccentricity and elliptical distortion cause timing and tracking errors. Compensation for residual imperfections is generally achieved by a precise servo control system which employs feedback from optically detectable indicia on the disk. The indicia, which are read by a laser stylus, usually include timing marks or pits for producing clocking signals, so-called wobble pits for tracking control and address pits for accessing information within the track. Typically the pits are arranged in various formats whereby the position of any particular pit has a specialized and singular meaning. These formats differ in accordance with the control system employed, the data rate, the laser spot size, the configuration of the servo feedback loop, data density and various other limiting factors. Unfortunately, differing formats are usually not compatible. Also, achieving a performance advantage in one format may require a tradeoff in another area. For example, it may be possible using a particular array of pits to accurately spot a disk position or address by track and sector. However, such an array may require undue disk space, and while access is accurate, it may be slow.
Compensation of angular and radial run out is traditionally done with feedback control based on media formats designed for feedback control. Feedback alone has its limitations and requires tight media tolerances, which add to the cost of the media. A feed forward, or predictor, system can relax the media tolerance requirements. Thus, the need exists for a media format that is designed for both feedback and feed forward control. Although a hardware system for implementing feed forward and feedback control may be more complex than one implementing feedback alone, such a system is more flexible and more powerful. Current technology allows for cost effective implementation of such complex servo designs.
In summary, there is a need for a simplified pit format for reducing the stringent physical tolerances required for currently available optical media without increasing access time. There is also a need for a pit format which has the capacity to allow for improved resolution as the technology improves.